1. Field of the Invention
The present invention relates generally to effluent scrubbers and more particularly to an improved scrubber inlet system for use with effluent scrubbers.
2. Brief Description of the Prior Art
In the semiconductor industry, exhaust gas scrubbers are used to cleanse the unreacted silicon gases that are exhausted from silicon deposition systems. These gas scrubbers typically operate by passing the exhausted gas through an environment permeated with a misted or atomized fluid, such as water, which reacts with the unreacted particles in the gas to form solid deposits which rain from the gas into a collection reservoir.
Cross-sectional views of prior art gas scrubbers having elements similar to some of the gas scrubber models manufactured by Shannon, Inc. and Airprotek, Inc. are illustrated in FIGS. 1a and 1b, respectively. Both of these scrubbers cleanse the silicon gas exhausted from the deposition system by reacting it with water from one or more scrubbing jets, so as to form solid Silicon Dioxide (SiO2).
The problem with these types of silicon gas scrubbers is that an SiO2 coating is constantly being formed within the inlet pipe and unwashed walls of the scrubbing chamber. For example, in both FIGS. 1a and 1b, gas 1 passes through the inlet pipe 2 into the open area or transition region 3, where some of the gas mixes with the fluid mist 4. Silicon gas 1, which has mixed with fluid mist 4, is then carried by gas turbulence 7 and diffusion back-up and deposited on the insufficiently washed inner walls of the scrubbing chamber, so as to form a build-up 5. This SiO2 build-up results in a significant problem which, until the present invention, remained unsolved.
As is shown in FIGS. 1a and 1b, the build-up of SiO2 results in the development of restrictions in the inlet flow pipe, as well as other places. These restrictions develop because the transition region between the inlet pipe and the scrubbing spray are dampened sufficiently so as to cause the silicon gas to react, but are insufficiently washed so as to prevent build-up. The transition region and inlet pipe are dampened for two reasons: first, because water mist from the scrubbing jet 6 is diffused back into the transition region 3 and entry pipe 2; and second, because an abrupt volume change occurs as gas exits the narrow inlet pipe and enters the large scrubbing chamber, thereby creating heavy gas turbulence 7 and causing water mist and precipitated solids to be swept back into the inlet pipe and onto the walls of the transition region.
The resulting restrictions cause back-pressure to be created in the reaction chamber of the silicon deposition system from which the gas is exhausted. Excessive back-pressure will result in loss of process control and require the deposition system to be shut down until the restriction can be removed. Silicon gas scrubbers presently require restriction removal between every 1 to 5 days, depending on specific conditions.
Restriction removal is performed by shutting the deposition system down, flushing the system and the scrubber out with nitrogen gas to make the surrounding environment safe to humans, unscrewing an access union 8 (FIG. 1b), mechanically plunging out the build-up, rinsing the inlet port with water, and then re-tightening the access union. In all, the average downtime for restriction removal is about 35 minutes: 10 minutes for a nitrogen gas pre-purge, 15 minutes devoted to the actual cleaning process, and 10 minutes for a nitrogen gas post-purge. At the present time, silicon deposition system downtime typically costs over $500.00 per hour.
One possible, but not completely effective, means of avoiding the solidification of SiO2 in the inlet port would be to use a venturi-type scrubbing system instead. Venturi scrubbers operate by passing uncleaned gas through a venturi and spraying or injecting water into the gas flow, thereby causing the solid particles in the gas to be impacted by the faster moving water droplets. This gas and water mixture is then passed into a cyclone or other apparatus which separates the dust-laden water droplets from the gas. The rapid and violent mixing of gas and fluid in venturi scrubbers, to some extent, helps to limit the amount of unclean gas which is exposed to unflushed walls.
Some examples of venturi-type scrubbers are shown in U.S. Pat. Nos. 3,620,510; 3,567,194; 3,638,925; 3,841,061; and 4,578,226. In these types of venturi scrubbers, the injected fluid is primarily used to scrub the incoming gas when forced through the venturi, and the fluid is given a rotational effect to improve the balance of the fluid/gas mixture. Since the overall purpose of a venturi scrubber is to inject cleaning fluid into the gas flow, no attempt is made by these scrubbers to introduce a mist free flow of fluid into the venturi. In addition, venturi scrubbers are generally not suitable for scrubbing silicon gas exhausted from silicon deposition systems because they develop restrictions rapidly and the venturi effect can adversely affect air pressure stabilization within the reaction chamber.
There are a number of other types of effluent gas scrubbing devices which utilize a centrifugal flow of fluid or gas to accomplish the scrubbing operation or to prevent particle adherence to internal surfaces of the scrubber. For example, U.S. Pat. No. 3,722,185, issued to Miczek, discloses a scrubber where water flows down the scrubbing chamber as a concentric sheet and tangentially introduced gas flows up the chamber in a helical path, such that dust particles, driven by centrifugal forces, are forced into the sheet of water and carried away. Use of a swirling sprayer within the scrubbing chamber (see FIG. 2) is disclosed in U.S. Pat. No. 2,281,254.
Another type of gas scrubber which uses a rotating film of fluid is the electro-inertial precipitator unit disclosed in Reif et al., U.S. Pat. No. 4,529,418. Reif et al disclose injecting a fluid film onto the inner surface of a collector tube through which gas passes. Electrostatic and centrifugal forces operating on the gas passing down the tube cause the dust particles within the gas to mix with the liquid and be washed away.
A system which is structurally similar to the present invention, but related to a non-analogous art, is the falling film heat exchanger disclosed in U.S. Pat. No. 2,545,028, issued to Haldeman. Haldeman discloses a heat exchanger in which a film of fluid adhering to the inlet pipe surface is used to absorb and retransmit heat between either two liquids, or a gas and a liquid, or perhaps even two gases. This patent does not concern itself with scrubbing uses, and is apparently not intended to prevent build-up of condensed material at the lower end of the inlet tube 17, (see FIG. 5).